1. Field of the Invention
This invention relates to the wet-process manufacture of phosphoric acid from phosphate ores and more particularly to the continuous wet-process manufacture of phosphoric acid in a reaction zone from a phosphate ore such as fluorapatite.
2. Description of the Prior Art
The conventional continuous procedures for the manufacture of phosphoric acid widely used today employ principally a reaction zone and a filtration zone. The reaction zone is composed of one or several reactors into which the raw materials are introduced. The raw materials often comprise one or more of phosphate ore, sulfuric acid, and recycled liquids coming from the washing of the solid residue formed in the continuous production of phosphoric acid. The reaction zone is furnished with a cooling system of more or less importance according to the type of reaction. For example, the differences observed in a reaction in hemihydrate, anhydrite or gypsum are demonstrated by the fact that these reactions take place at different temperatures and accordingly, produce variable amounts of heat energy, which must be eliminated. Cooling is most often performed by evaporation in the conventional processes. One specific type of cooling system places the reaction zone in a partial vacuum, another type utilizes the treatment in partial vacuum of a limited flow of the reaction medium.
Air cooling provides yet another cooling method while eliminating the vacuum apparatus. Several of the different modes of cooling have been described notably in "PHOSPHORIC ACID" by A. V. Slack, 1968 edition, pp. 227-31, and reference is made thereto for a more detailed description of the prior art.
In the case of air cooling, air is circulated at and over the surface of the reaction zone so that it becomes laden with moisture and carries off the excess heat energy. Various gaseous products may also be given off into the circulating air. This system, of which the present invention constitutes an improvement, necessarily includes an apparatus designed for scrubbing the air containing the evaporated moisture and products before it is discharged into the atmosphere. The scrubbing operation permits the elimination of fluorous compounds derived from the phosphate ore undergoing reaction, principally hydrofluoric acid HF, and silicon tetrafluoride SiF.sub.4. These compounds are released in varying proportions, along with water vapor and the carbon dioxide during the leaching reaction on the phosphate ore.
In addition to the reaction zone with its cooling system, the conventional apparatus commonly employs a filtration zone, one or several filters are often provided in the filtration zone and enable the operation of methodic washing methods. A methodic washing with a filter divided into sectors is accomplished in the following manner: after having collected the strong acid which constitutes production, the calcium sulfate is washed to one side by water, picking up minor quantities of phosphoric acid and becoming a very weak acid; the very weak acid is recycled and so on, until medium-strength acid is achieved and then returned to the reaction zone.
In order to produce concentrated phosphoric acid, the water necessary for the complete reaction-filtration system is carefully measured and distributed in order to fulfill the different water requirements, especially the requirement of water needed for wetting of the ore, dilution of the sulfuric acid, methodic washing of the calcium sulfate, as well as scrubbing of the cooling air. The water is commonly called "process-water". Now, frequently the washing of the air increases the amount of water consumed and leads to large amounts of water representing unusable effluents which must sometimes be neutralized before disposal.